1. Field of the Invention
The present invention relates to a high-frequency package for mounting a high-frequency circuit component of a high-frequency semiconductor device, a high-frequency circuit or the like used in a high-frequency waveband.
2. Description of the Related Art
A high-frequency package used for accommodating a high-frequency circuit component of a high-frequency semiconductor device, a high-frequency circuit or the like that operates in a high-frequency waveband such as a MHz band or a GHz band is, for example, a one shown in FIGS. 11A to 11C disclosed in Japanese Patent Publication No. 2605502.
FIGS. 11A to 11C are views showing an example of such a conventional high-frequency package, FIG. 11A is a plan view showing by cutting away, FIG. 11B is a sectional view taken on line I—I of FIG. 11A, and FIG. 11C is a bottom view. Moreover, FIG. 12 is a perspective view showing by magnifying an essential portion of the high-frequency package shown in FIGS. 11A to 11C. In these views, reference numeral 1 denotes a package substrate made of ceramics or the like, and reference numeral 2 denotes a package side wall made of ceramics or ceramics with a surface metalized, which is mounted on the surface of the package substrate 1. The upper end face of the package side wall 2 that is not in contact with the package substrate 1 is sealed by a lid 3 made of metal such as gold or alloy such as iron-nickel-cobalt alloy. Reference numeral 4 denotes a die bonding region made by applying metalization 5 to the surface of the package substrate 1, reference numeral 6 denotes a dielectric substrate made of ceramics or the like, an internal high-frequency transmission line 19 made by a metal thin film is formed on the surface of the dielectric substrate 6, and the die bonding region 4, the dielectric substrate 6 and the internal high-frequency transmission line 19 are constituted in a cavity surrounded by the package substrate 1, the package side wall 2 and the lid 3. Moreover, a grounding metal thin film 8 and a signal line metal thin film 9 are formed on the bottom portion of the package substrate 1, an external coplanar line 10 is constituted by them, and the signal line metal thin film 9 is structured so as to be electrically connected to the internal high-frequency transmission line 19 by a via hole 11 made of metal.
Reference numeral 18 denotes a grounding metal thin film formed on the surface of the dielectric substrate 6, which constitutes an internal coplanar line 20 together with the internal high-frequency transmission line 19. Moreover, reference numeral 12 denotes a grounding via hole that electrically connects the grounding metal thin film 8 of the external coplanar line 10 and the grounding metal thin film 18 of the internal coplanar line 20.
As shown by a sectional view in FIG. 13, this high-frequency package is surface-mounted on a circuit board 13 made of glass epoxy, fluorine resin, ceramics or the like, and a high-frequency circuit is constituted.
Since the conventional high-frequency package is constituted as described above, there is a problem that when this high-frequency package is surface-mounted on the circuit board 13, a ground network path between the grounding metal thin films 8 becomes long, an inductance component increases, and therefore, a grounding state of the grounding metal thin films 8 tends to become unstable in a bonding region between the package substrate 1 and the circuit board 13 in a case where a gap between the grounding metal thin films 8 placed on both the sides of the signal line metal thin film 9 is longer than one half of a wavelength of used high-frequency signals, with the result that characteristic impedance changes and a reflection loss of high-frequency signals is caused locally in this region, a shield effect of high-frequency signals becomes insufficient and a radiation loss is caused in this region, and therefore, a transmission characteristic of high-frequency signals is deteriorated by these losses.
Furthermore, since the conventional high-frequency package is constituted as described above, there is a problem that when this high-frequency package is surface-mounted on the circuit board 13, because of generation of a stray capacitance component inside the package substrate 1 existing between the signal line metal thin film 9 and the grounding metal thin films 18, 8 and a stray capacitance component inside a dielectric substrate of the circuit board 13 existing between the signal line metal thin film 9 and a lower-face-side grounding conductor (not shown in the drawings) of the circuit board 13 when this high-frequency package is surface-mounted on the circuit board 13, a capacitance component existing between the package substrate 1 and the circuit board 13 increases, with the result that high-frequency mismatching is caused at input/output portions of the high-frequency package at the time of surface-mounting on the circuit board 13, the loss of high-frequency input/output signals increases and a voltage standing wave ratio (abbreviated as VSWR hereafter) deteriorates.
Besides, the conventional high-frequency package has a problem that when this high-frequency package is surface-mounted on the circuit board 13, because a grounding conductor that electrically connects the grounding metal thin film 18 on the upper face of the package substrate 1 and the grounding metal thin film 8 on the lower face is not disposed, the grounding metal thin film 18 and the grounding metal thin film 8 are not in high-frequency connection, and therefore, a grounding state becomes unstable, with the result that the loss of high-frequency input/output signals increases at the input/output portions of the package substrate 1 and a VSWR tends to deteriorate. Moreover, there is a problem that since a propagation mode of high-frequency signals becomes discontinuous at the input/output portions of the package substrate 1, a radiation loss increases, and consequently, a transmission characteristic of high-frequency signals at the input/output portions of the high-frequency package deteriorates.